What does that MMS button on my social sharing mean? To put it simply: the difference between a text link and a MMS is the user experience and how the sharing analytics are tracked.
When a user receives a text message with the link, they will be directed to the gallery. From there they can download the media to their device or share it to social media. The dashboard will then report the sharing analytics and track where it ends up.
With MMS, the user will be able to download the media straight from their text - without having to access the gallery through the link. While this makes for a quicker user-experience, MMS social sharing cannot be tracked on the dashboard and will only show that they texted the media.
Inside the Collection click the Settings gear in the top right corner - Sharing - scroll down to the SMS section - click the circle next to "Send media as a MMS attachment".